This invention relates to methods of inhibiting nuclear spin relaxation of hyperpolarized noble gas. More particularly, the invention relates to a method of inhibiting depolarizing interactions of hyperpolarized .sup.129 Xe with a polymeric coating on a surface.
The number and variety of applications of noble gases, particularly .sup.3 He and .sup.129 Xe, polarized through spin-exchange optical pumping (Bhaskar N D, Happer W, and McClelland T, Phys Rev Lett 49:25 (1982); Happer W, Miron E, Schaefer S, van Wijngaarden, and Zeng X, Phys Rev A 29:3092 (1984)) have grown rapidly over the past few years. Most recently, the enhanced NMR signals of laser-polarized .sup.129 Xe, which are about five orders of magnitude larger than those from thermally polarized .sup.129 Xe, have made possible the first high-speed biological magnetic resonance imaging (MRI) of a gas (Albert M S, Cates G D, Driehuys B, Happer W, Saam B, Springer C S, and Wishnia A, Nature 370:188 (1994)), opening many new avenues of research. Historically, polarized .sup.129 Xe has been used for fundamental symmetry studies (Chupp T E, Hoare R J, Walsworth R L, and Wu B, Phys Rev Lett, 72:2363 (1994)), nuclear spin relaxation studies of solids. (Gatzke M, Cates G D, Driehuys B, Fox D, Happer W, and Saam B, Phys Rev Lett, 70:690 (1993)), high resolution nuclear magnetic resonance spectroscopy (NMR) (Raftery D, Long H, Meersmann T, Grandinetti P J, Reven L, and Pines A, Phys Rev Lett, 66:584 (1991)), and cross-polarization to other nuclei (Gatzke et al., Phys Rev Lett, 70:690 (1993); Driehuys B, Cates G D, Happer W, Mabuchi H, Saam B, Albert M S, and Wishnia A, Phys Lett A, 184:88 (1993); Long H W, Gaede H C, Shore J, Reven L, Bowers C R, Kritzenberger J, Pietrass T, Pines A, Tang P, and Reimer J A, J Am Chem Soc, 115:8491 (1993)). Polarized .sup.3 He is an important nuclear target (Anthony P L, et al., Phys Rev Lett, 71:959 (1993); Middleton H, PhD Thesis, Princeton University, unpublished; Newbury N R, et al., Phys Rev Lett, 67:3219 (1991); Newbury N R, et al., Phys Rev Lett, 69:391 (1992)) and has also been shown to be an excellent nucleus for gas-phase MRI (Middleton H, et al., Magnetic Resonance in Medicine, 33:271 (1995)).
All of these applications require that the highly non-equilibrium polarizations of the noble gas nuclei be long-lived, i.e., the decay of polarization to thermal equilibrium level must be slow. However, interactions of the polarized noble gas nuclei with surfaces can cause rapid relaxation, often resulting in relaxation times T.sub.1 that are undesirably short. Understanding these mechanisms, and devising methods of inhibiting relaxation, is vital for continued progress in a large variety of experiments using polarized noble gases.
Bouchiat and Brossel identified relaxation of hyperpolarized rubidium on coatings of paraffin on the walls of glass resonance cells (Bouchiat M A, and Brossel J, Phys Rev, 147:41 (1996)). This relaxation was attributed to adsorption of rubidium on the coatings leading to depolarizing interactions such as dipole-dipole interaction between electron spin of the rubidium atom and the nuclear spin of the protons in the coating. This paper reported a diminution of such interactions upon substituting (CD.sub.2).sub.n paraffins for (CH.sub.2).sub.n paraffins, i.e., deuterating the paraffins. Bouchiat and Brossel, however, do not extrapolate on this work and make no inferences concerning potential interactions of other elements with paraffins. Nor does this paper indicate whether any other polymeric materials depolarize rubidium.
Zeng and co-workers made substantial progress in reducing .sup.129 Xe surface relaxation by introducing the use of a silicone coating agent (Zeng X, Miron E, van Wijngaarden W A, Schreiber D, and Happer W, Phys Lett, 96A:191 (1983)). Relaxation times of order T.sub.1 .about.20 min are now routinely attained using such coatings. Nonetheless, these relaxation times are still approximately two orders of magnitude shorter than what is ultimately possible for gaseous .sup.129 Xe at standard temperatures and pressures. It has been thought that continuing inability to improve nuclear spin lifetimes is attributable to paramagnetic impurities in the coating compositions. Efforts to reduce relaxation by removing such impurities, however, have met with little success. Accordingly, it is evident that better understanding of the .sup.129 Xe surface interactions has been needed.
As a result, there exists a need for improving the yield of noble gas hyperpolarization processes by reducing the depolarizing interactions of the noble gas with surfaces in the hyperpolarization system.
There is also a need for increasing the total amount of hyperpolarization in a noble gas by reducing counteracting depolarizing interactions between the noble gas and its physical environment.
Moreover, there is a need for improving the duration of storage of hyperpolarized noble gas by reducing depolarizing interactions of the noble gas with storage containers which house the gas.
In addition, there is a need for improving the efficiency of magnetic resonance imaging methods which require the use of hyperpolarized noble gas nuclei by decreasing the amount of physical interaction of the noble gas with physical systems employed for delivery of the hyperpolarized gas for imaging.